Nanowire photodetector and image sensor with internal gain

ABSTRACT

A 1D nanowire photodetector device includes a nanowire that is individually contacted by electrodes for applying a longitudinal electric field which drives the photocurrent. An intrinsic radial electric field to inhibits photo-carrier recombination, thus enhancing the photocurrent response. Circuits of 1D nanowire include groups of photodetectors addressed by their individual 1D nanowire electrode contacts. Placement of 1D nanostructures is accomplished with registration onto a substrate. A substrate is patterned with a material, e.g., photoresist, and trenches are formed in the patterning material at predetermined locations for the placement of 1D nanostructures. The 1D nanostructures are aligned in a liquid suspension, and then transferred into the trenches from the liquid suspension. Removal of the patterning material places the 1D nanostructures in predetermined, registered positions on the substrate.

PRIORITY CLAIM AND REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 from priorprovisional application Ser. No. 60/903,633, which was filed on Feb. 27,2007; and from prior provisional application Ser. No. 60/903,750, whichwas filed on Feb. 27, 2007.

STATEMENT OF GOVERNMENT INTEREST

This application was made with Government support under ECS-0506902awarded by the National Science Foundation and under N00014-05-1-0149awarded by the Office of Naval Research. The Government has certainrights in this invention.

FIELD

A field of the invention is photodetection and image sensing. Theinvention concerns photodetector and image sensors that convert opticalsignals into electrical signals.

BACKGROUND

Any electronic device that detects and/or processes optical signals mustconvert the sensed signals to electrical signals. This is accomplishedwith a photodetector. Image sensors include a spatial arrangement ofphotodetectors (pixels) that can be used to record and reconstruct animage. Image sensors are used in a wide variety of applications, e.g.,toys, games, cameras, medical equipment, security equipment, processmonitoring, portable handsets, personal digital assistants, scientificinstruments, etc. The modern types of image sensors include chargecoupled devices (CCDs) and CMOS (Complementary Metal OxideSemiconductor) image sensors (also referred to as active pixel sensors).

A CCD sensor includes an array of linked, or coupled, light-sensitivecapacitors. A CCD gets its name from the way the charges on its pixelsare read after an exposure. After the exposure the charges on the firstrow are transferred to a place on the sensor called the read outregister. From there, the signals are fed to an amplifier and then on toan analog-to-digital converter. The CCD shifts one whole row at a timeinto the readout register. The readout register then shifts one pixel ata time to the output amplifier.

CMOS image sensors are fabricated on semiconductor substrates, using theCMOS fabrication process used to manufacture computer processors andmemories. Pixels in CMOS image sensors have their own charge-to-voltageconversion. In a typical CMOS pixel there is a photodetector, typicallya photodiode or photogate, and a number of transistor devices. Thephotodetector can be reset when it is effectively connected to the powersupply through a reset transistor. Another transistor typically acts asa buffer and allows the pixel voltage to be observed without removingthe accumulated charge. A row-select transistor is a switch that allowsa single row of the pixel array to be read by read-out electronics. In atypical CMOS image sensor, the pixels are arranged in a two-dimensionalrow and column arrangement. Pixels in a given row share reset linespermitting a row to be reset. Pixels are also selected by row. Outputsof each pixel in any given column are tied together. As one row isselected at a time no competition for the output line occurs. Furtheramplifier circuitry is typically on a column basis. The CMOS imagesensor itself typically includes integrated circuits, e.g., columnamplifier circuitry and read out electronics, which permit the CMOSimage sensor to output digital bits.

CCDs were once considered the benchmark for obtaining the highest imagequality in demanding applications such as medical imaging and digitalphotography. Compared to early CMOS image sensors, CCDs had betteruniformity, and could provide greater resolution and fill factor.However, as the feature size of CMOS fabrications has been reduced, CMOSimage sensors have improved to the point that they can be used indemanding imaging applications.

Photodetector and photogates in CMOS images sensors are fabricatedthrough ion implantation, etching, deposition, etc. processing steps.CMOS image sensors are more likely than CCD images sensors to sufferfrom fixed-pattern and dark-current noise. CCDs also tend to havesuperior dynamic range. CMOS image sensors can generally be manufacturedless expensively. In addition, most image-sensor support circuitry isCMOS based, so it can be integrated on the same chip as a CMOS imagesensor, which lowers overall system cost and size. Also, CMOS imagesensors do not require multiple voltages for readouts as do CCDs, sothey typically consume only a fraction of the power of a comparable CCDimage sensor. Further improvements in CMOS style sensors could have asignificant positive impact on devices that make use of them.

Efforts have been directed toward the use of nanowires asphotodetectors. Nanowires have been recognized as having the potentialto be highly sensitive photodetectors and could represent a greatadvance in CMOS image sensors. However, the incorporation of nanowiresas photodetectors in practical CMOS integrations has proven difficult.Additionally, the photon absorption, gain and current generation innanowires are not fully understood.

For example, under UV illumination, it has been observed thatphotogenerated holes in ZnO nanowires oxidize surface oxygen species.This transient response of nanowires or how to control it is not fullyunderstood. See, e.g., Lu et al., “Ultraviolet Photodetectors with ZnONanowires Prepared on ZnO:Ga/Glass Templates”, App. Phys, Lett. 89,153101 (2006). Others have observed the oxygen sensitivity of ZnOnanowires, which may be used, for example, for gas sensing applications.See, Fan et al, “ZnO Nanowire Field Effect Transistor and Oxygen SensingProperty”, Applied Physics Letters 85, 5932 (2004).

Another issue in making practical use of nanowires as photodetectorsinvolves the placement of nanowires and connecting into integratedcircuits. While nanowires have been used in groups to realizephotodetection and can be deposited in parallel by the standardLangmuir-Blodgett technique for this purpose, the registered placementand registration of nanowires necessary for complex image sensorcircuits is lacking.

Fluidic assisted alignment, electrical (electrophoresis) and magneticfield guided alignment, and Langmuir-Blodgett technique, have been usedpreviously to assemble nanowires on the surface of liquids in awell-aligned fashion, similar to nematic phase liquid crystals, andconsequently transferred to the surface of solid substrates whilemaintaining their alignment/organization.

These techniques do not provide for the registered placement of a 1Dnanostructure, however. The Langmuir-Blodgett technique as used in theart does not allow the precise control resulting nanowire location andthe registration of nanowires on a substrate. Similarly, fluidicalignment cannot achieve precise control of the nanowire location andregistration on a substrate, and also cannot be used with largesubstrates, limiting its applicability to very small scales. On theother hand, the electrophoretic technique does not work on the largescale and requires applying an electric or magnetic field to guide thenanowire assembly.

However, microbeads and nanoparticles have been placed precisely on asubstrate by using either the Langmuir-Blodgett technique orself-assembly techniques combined with photolithography to predefine thepockets where the nanoparticles are going to be positioned. See Yin, et.al., J. Am. Chem. Soc. 2001, 123, 8718 and Cui, et. al. Nano Letters 4(6); 1093-1098 (2004).

SUMMARY OF THE INVENTION

The invention provides a practical 1D nanowire photodetector with highgain that can be controlled by a radial electric field established inthe 1D nanowire. A 1D nanowire photodetector device in embodiments ofthe invention includes a nanowire that is individually contacted byelectrodes for applying a longitudinal electric field that drives thephotocurrent. An intrinsic electric field in the radial direction of thenanowire inhibits photo-carrier recombination, thus enhancing thephotocurrent response. The invention further provides circuits of 1Dnanowire photodetectors, with groups of photodetectors addressed bytheir individual 1D nanowires electrode contacts. The invention alsoprovides a method for placement of 1D nanostructures, includingnanowires, with registration onto a substrate. A substrate is coatedwith patterning material, e.g., photoresist, and trenches are formed inthe patterning material at predetermined locations for the placement of1D nanostructures. The 1D nanostructures are aligned in a liquidsuspension, and then transferred into the trenches from the liquidsuspension. Removal of the patterning material leaves the 1Dnanostructures in predetermined, registered positions on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate a 1D nanowire photodetector 10 of the inventionwith high internal gain;

FIG. 2 illustrates a pixel or sub pixel of a preferred embodiment imagesensor that uses a 1D nanowire photodetector;

FIGS. 3A-3D illustrate preferred embodiment 1D nanowire photodetectordevices for RGB color sensing of the invention;

FIG. 4 illustrates a preferred embodiment method for placement of 1Dnanowires with predetermined registration;

FIG. 5 illustrates another preferred embodiment method for placement of1D nanowires with predetermined registration; and

FIG. 6A shows a vertical single nanowire photodetector, and FIG. 6Bshows a vertical nanowire photodetector array, and FIG. 6C shows aphototransistor device according to additional embodiments of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a practical 1D nanowire photodetector with highgain that can be controlled by a radial electric field established inthe 1D nanowire. A 1D nanowire photodetector device of the inventionincludes a nanowire that is individually contacted by electrodes forapplying a longitudinal electric field which drives the photocurrent. Anintrinsic radial electric field to the nanowire inhibits photo-carrierrecombination, thus enhancing the photocurrent response. The inventionfurther provides circuits of 1D nanowire photodetectors, with groups ofphotodetectors addressed by their individual 1D nanowires electrodecontacts.

1D nanowire photodetectors with internal gain of the invention can berealized with different materials, including group IV, III-V, II-VIsemiconductors. A preferred 1D nanowire photodetector is a ZnO nanowirewith a radial doping profile that inhibits hole and electronrecombination. Embodiments of the invention provide the ability tointegrate group IV, III-V, II-VI semiconductor nanowires asphotodetectors with current CMOS image sensor technology for highersensitivity, higher resolution and lower power consumption imagesensors.

In preferred methods for controlling gain of the invention, a radialelectric field is created in a 1D nanowire that inhibits hole andelectron recombination. The inhibition of recombination increasescarrier lifetime and produces high photoconductive gain 1D nanowirephotodetectors. Gain control in the invention can be realized via thematerial structure in a 1D nanowire, such as having a core portion and ashell portion of the nanowire doped differently or have heterostructureswith different composition, for example. Additionally, a passivationouter layer on the nanowire can act to control the radial fields thatinhibit carrier recombination during stimulation by incident light. Theinhibition of recombination increases the sensitivity of the nanowirephotodetector significantly and results in internal gain. Thephotoconductive gain of a photodetector is given by the ratio of thecarrier lifetime and the carrier transit time in the active area of thedevice.

The invention also provides circuits of 1D nanowire photodetectors thatare individually addressed. Registered placement for the necessaryregistration of nanowires to achieve circuits of individual 1D nanowirephotodetectors is provided.

The invention provides a method for placement of 1D nanostructures withregistration. Example nanostructures include nanowires, nanotubes, andnanobelts, etc. Embodiments of the invention add precise registration toplace nanostructures in a predetermined position on a substrate, andmake use of prior alignment techniques for placement of nanostructures,e.g., fluidic assisted alignment, electrical (electrophoresis), magneticfield guided alignment, Langmuir-Blodgett technique, and contacttransfer methods. The invention allows assembly and transfer of 1Dnanostructures to predetermined positions on a substrate (such as awafer), and therefore allow integration to existing devices in CMOScircuits, Si photonic chip, MEMS system, etc.

Embodiments of the invention provides for precise control of theplacement location for 1D nanostructures, by combining a trenchregistration placement with any number of prior nanowire alignmenttechniques. Prior alignment techniques that can be used with the trenchregistration of the invention include but are not limited to the fluidicassisted alignment, electrical (electrophoresis), magnetic field guidedalignment, the Langmuir-Blodgett technique, and the contact transfermethod.

A preferred embodiment of the invention provides for the transfer andthe controlled assembly of nanowires onto a substrate using an alignmenttechnique, e.g., preferably the Langmuir-Blodgett technique, withregistration of the nanowires facilitated by lithographically patternedtrenches. The precise placement of nanowires can facilitate their use inlarge scale circuit integrations, such as functional nanodevices,circuit interconnections, etc. in a manner that is compatible with theexisting CMOS technology, MEMS technology, Si photonics, etc.

Preferred embodiments of the invention will now be discussed withrespect to the drawings. The drawings may include schematicrepresentations, which will be understood by artisans in view of thegeneral knowledge in the art and the description that follows. Featuresmay be exaggerated in the drawings for emphasis, and features may not beto scale.

1D Nanowire Photodetector Devices and Photoconductive Gain Control inNanowires

High internal gain in 1D nanowire photodetectors of the invention isachieved by control of the internal radial electrical field to inhibithole and electron recombination and to promote charge carrier flow inthe core of a nanowire in response to photons absorbed by the nanowiresunder an applied longitudinal electric field. The internal electricalfield in the 1D nanowires of the invention from centroid to surface dueto band bending separate photo-generated electron/holes to nanowiresurface and center, which enhances the photodetector efficiency byreducing the recombination of electrons and holes. 1D photodetectordevices and circuits of the invention have individual nanowires (i.e.,1D) as the detector, and the individual nanowires can be arranged incircuits. Color photodetector devices are also provided.

The high density of surface states in semiconductor nanowires can trapthe photogenerated carriers (for example, holes in ZnO nanowires, astrapped carriers) and the unpaired free carriers are collected byelectrodes. Therefore, the free carrier lifetime is extremely long dueto the trapping of the photogenerated carriers, and also the free chargecarrier transit time can be very short due to the small physicaldimensions of the nanowires. The combination of these two propertiesresults in extremely high photoconductive gain, and hence extremely highsensitivity, up to 10,000 times greater than a state-of-the-artcommercial InGaAs PIN diode detector.

Visible-blind ZnO nanowire photodetectors with internal photoconductivegain as high as G˜10⁸ have been fabricated and characterized inaccordance with the invention. The photoconduction mechanism in thesedevices has been verified over a wide temporal domain, from 10⁻⁹ to 10²seconds, revealing the coexistence of fast (τ˜20 ns) and slow (τ˜10 s)components of the carrier relaxation dynamics. The extremely highphotoconductive gain is attributed to the presence of oxygen relatedhole-trap states at the nanowire surface, which prevents charge-carrierrecombination and prolongs the photocarrier lifetime, as evidenced bythe sensitivity of the photocurrrent to ambient conditions.Surprisingly, this mechanism appears to be effective even at theshortest time scale investigated of t<1 ns. Despite the slow relaxationtime, the extremely high internal gain of ZnO nanowire photodetectorsresults in gain-bandwidth products higher than GB˜10 GHz.

The invention has identified that photocarrier relaxation dynamicsconsist of a fast decay component, in the nanosecond time range, whicharises from the fast carrier thermalization and hole-trapping at deepsurface states, followed by a persistent photocurrent which decayswithin several seconds. The persistent photocurrent is leveraged in 1Dnanowire photodetectors of the invention. This model is readilygeneralized to the case of other low-dimensional 1D semiconductors wherethe high density of surface trap states enhances the photocarrierlifetime.

FIGS. 1A-1C illustrate a 1D semiconductor nanowire photodetector 10 ofthe invention with high internal gain. The photodetector consists of anindividual semiconductor nanowire 12 having a free carrier core 14 andtrapped carrier shell 16. The core 14 and shell 16 can be created, forexample, by doping that varies in the radial direction of the nanowire12. The nanowire 12 is individually contacted by electrodes 18 thatpermit an applied bias voltage 20 to create a longitudinal electricfield in the nanowire 12 that drives free photogenerated carriers to becollected at electrodes 18. Upon exposure to radiation hv, andapplication of the bias voltage, high internal photoconductive gain isrealized with inhibition of photocarrier recombination.

FIG. 1B shows the 1D nanowire photodetector device 10 in dark. The banddiagram indicates the trap states at surfaces and the intrinsic electricfields from centroid to surface. FIG. 1C shows the 1D nanowirephotodetector under light. The band diagram indicates the surface statestrap holes in the shell 16, leaving behind the electrons in the core 14,which contribute to the photocurrent collected at electrodes.

Embodiments of the invention provide a number of advantages. The highsensitivity of nanowire photodetectors and the low voltage operationenables lowering the operating voltage of providing for simplifiedcircuitry, reducing the manufacture complexity, decreases the pixelsizes, and lowers power consumption.

FIG. 2 illustrates a portion of an integrated circuit that is based upona nanowire photodetector device 10 of the invention. Because the 1Ddevice can be individually contacted by electrodes 18 and registeredprecisely on a substrate, it can be integrated with conventional CMOSrow 22 and column 24 circuitry (For example, see Hsiu-Yu Cheng andYa-Chin King, IEEE TRANSACTIONS ON ELECTRON DEVICES, 50(1), 91 (2003)).While FIG. 2 illustrates a single pixel (or sub pixel of one color)photodetector, artisans will appreciate that is readily replicable toform a large array. Gain with a 1D nanowire photodetector 10 of theinvention can be high enough that it can be possible in applications toomit an op amp 26 typically used in CMOS image sensors. As seen in FIG.2, the 1D nanowire photodetector 10 of the invention can be preciselyregistered on a wafer and individually contacted to replace conventionalp-i-n photodiodes used in current commercial CMOS image sensors.

An additional advantage provided by the 1D nanowire photodetector 10 isreduced power consumption, which is a key parameter in any portableelectronic device. The 1D nanowire photodetector maintains highsensitivity at very low operational voltage to permit reduced powerconsumption by lowering of operating voltage of imaging devices. Also,the nanowires can be as top mounted to the CMOS circuit surface and thewafer real estate occupied by a 1D nanowire photodetector 10 and itselectrode contacts 18 can be smaller than that of a convention p-i-nphotodiodes, providing the ability to reduce pixel (and sub pixel) size.

FIGS. 3A-3D illustrate different embodiment devices for RGB colorsensing with 1D nanowire photodetectors of the invention. In FIG. 3A,three separate 1D nanowires 12 a, 12 b, 12 c, are aligned and registeredwith respective red, green and blue color filters 28 a, 28 b, 28 c, andcontacted by separate sensing electrodes 18 a, 18 b, 18 c and a commonelectrode 30. In FIGS. 3B-3D, separate color filters are not necessarybecause nanowires or portions or nanowires are made color sensitive.This can be accomplished with nanowires of different materials and hencedifferent band gaps, or with nanowires of the same material that arefunctionalized with different nanoparticles, organic dyes, polymers,etc. In FIG. 3B the three separate 1D nanowires 12 a, 12 b, and 12 cthat are aligned and registered are respectively sensitive to red, greenand blue wavelengths. In FIG. 3C, three separate 1D nanowire axialsegments 12 a, 12 b, and 12 c (part of the same nanowire, or axialnanowire heterostructures) are respectively sensitive to red, green andblue wavelengths. In FIG. 3D, three separate 1D nanowire radial segments12 a, 12 b, and 12 c (part of the same nanowire, or radial nanowireheterostructures, radial quantum well structures, multiple quantum wellstructures, superlattices, etc.) are respectively sensitive to red,green and blue wavelengths.

1D nanowire photodetectors placed in predetermined registered positionsand individually contacted by electrodes were tested. In preliminarytesting of the invention, high photoconductive gain (up to G˜10⁸) andhigh gain-bandwidth product (up to GB˜10 MHz) have been demonstrated inZnO and InP nanowire photodetectors. Other photosensitive nanowirematerials are expected to demonstrate the high internal gain with radialfields applied to inhibit photogenerated carrier recombination.

The performance of the 1D nanowires is strongly influenced by highsurface-to-volume ratio trapping at surface states, which drasticallyaffects the transport and photoconduction properties of nanowires. Inthe presence of a high density of hole-trap states at the nanowiresurface, upon illumination with photon energy above the bandgap (Eg),electron-hole pairs are photogenerated and holes are readily trapped atthe surface, leaving behind unpaired electrons which increase theconductivity under an applied electric field.

It has been previously shown that in ZnO thin films and nanowires thatoxygen gas is adsorbed on the oxide surface and captures the freeelectrons present in the n-type oxide semiconductor [O₂(g)+e⁻→O₂ ⁻(ad)].A low-conductivity depletion layer is formed near the surface; uponillumination at a photon energy above Eg, and electron-hole pairs arephotogenerated [hv→e⁻+h⁺]; holes migrate to the surface along thepotential slope produced by band bending and discharge the negativelycharged adsorbed oxygen ions [h⁺+O₂ ⁻(ad)→O₂(g)] and consequently oxygenis photodesorbed from the surface.

Under an electric field, the unpaired electrons destruct the depletionlayer and increase the conductivity, until oxygen gas adsorbed at thesurface is ionized and produces holes that can recombine with theunpaired electrons. This mechanism of trapping through oxygen adsorptionand desorption in ZnO nanowires augments the high density of trap statesusually found in nanowires due to the dangling bonds at the surface,thus enhancing the photoresponse.

In a nanowire, at low incident light intensities, the photocurrentincreases linearly with light intensity, consistent with the chargecarrier photogeneration efficiency proportional to the absorbed photonflux, while at higher light intensities it deviates below thislinearity. The sublinear dependence of the photocurrent on lightintensity can be understood assuming that at higher light intensitiesthe number of available oxygen hole-traps present at the surface isincreasingly reduced, leading to the saturation of the photoresponse. Inthis case, the density of free carriers in the nanowire can be expressedas:

$\begin{matrix}{n = {\frac{1}{AL}\frac{F}{1 + \sqrt{F/F_{0}}}T_{i}}} & (1)\end{matrix}$

where F is the photon absorption rate, F₀ is the photon absorption rateat which trap saturation occurs, A and L are the nanowire cross sectionand length, respectively, and T₁ is the carrier lifetime (related to thehole-trapping, oxygen desorption and adsorption mechanism). From theusual expression of the photocurrent (I_(ph)), therefore:

$\begin{matrix}{I_{ph} = {{qnvA} = {{q\left( \frac{T_{i}}{T_{t}} \right)}\frac{F}{1 + \sqrt{F/F_{0}}}}}} & (2)\end{matrix}$

where q is the elementary charge, n is given by Equation 1 and μV/1 isthe carrier drift velocity. The best fit to the data obtained byEquation 2 from which F₀=1.4×10⁶ s⁻¹ has been deduced. The radialelectric fields that inhibit free and trapped carrier recombinationperform a function that is similar to prior photoconductors withblocking contacts, i.e. with a Schottky barrier at the metalelectrode-semiconductor interface, which can exhibit hole-trapping inthe reversed-bias junction that shrinks the depletion region and allowstunneling of additional electrons into the photoconductor; if electronspass multiple times, this mechanism yields photoconductive gain greaterthan unity. Similarly, suppressed recombination of charge carriers hasalso been reported in p-i-n diodes with blocking contacts and type IIheterojunctions, where the increase of photoresponse times results inlarge photoconductive gain. The radial electric fields created in 1Dnanowire photodetectors of the invention performs a similar function.For example, in a 1D ZnO nanowire photodetector of the invention, holesare efficiently trapped at surface states and multiple electrons passingthrough the nanowire can lead to photoconductive gain. The gain isdefined as the ratio between the number of electrons collected per unittime and the number of absorbed photons per unit time (G=N_(el)/N_(ph));from Equation 2 it follows:

$\begin{matrix}{G = {\frac{I_{ph}}{cF} = {\left( \frac{T_{i}}{T_{t}} \right)\frac{1}{1 + \sqrt{F/F_{0}}}}}} & (3)\end{matrix}$

where the first term on the right-hand side is the usual expression forthe gain (the ratio between carrier lifetime and carrier transit time)and the second term accounts for trap saturation at high excitationintensities.

The increased photocarrier lifetime due to the presence of surfacestates, combined with the decreased carrier transit times due to thereduced dimensionality of the 1D nanowire photodetector devices of theinvention, i.e., the small spacing between the electrodes, results inphotoconductive gain as high as G=2×10⁸. As a nonlimiting example,spacing can vary from few tens of nm (lithography limited) to fewmicrons, depending on the active area that one wants to achieve.Reducing the electrode spacing reduces the carrier transit time (henceenhances the gain), but also reduces the light collection area and thussensitivity.

To determine the charge carrier lifetime, T₁, photocurrent relaxation bytime-resolved measurements in 1D nanowire photodetectors of theinvention was tested. Photocurrent rise upon continuous illumination wasmeasured and the photocurrent decay after removal of incident light, atdifferent applied bias voltages. The testing revealed that photocurrentdynamics is independent of the sign and intensity of the externalelectric field throughout the whole range of applied fields investigatedless than 5V applied. From the best fit to the data obtained by adouble-exponential rise and decay functions, a weight-averagedphotocurrent rise and decay time constants of τ_(rise)=23 s andτ_(decay)=33 s was determined. From the conventional expression for the3 dB bandwidth of a photodetector, B=½πT₁, and the experimental value ofthe carrier lifetime (Tl=33 s) calculations for the ZnO nanowiresprovide B˜5×10−3 Hz. Considering Equation 3, the gain-bandwidth productwill be given by:

$\begin{matrix}{{GB} = {\left( \frac{1}{2\pi\; T_{t}} \right)\frac{1}{1 + \sqrt{\frac{F}{F_{0}}}}}} & (4)\end{matrix}$

which accounts also for hole-trapping saturation at high excitationinfluences. Despite the slow photocurrent relaxation time, the high gainvalues result in large gain-bandwidth products, implying that asignificant photo response is expected in the 1D nanowire photodetectorsof the invention even at high modulation frequencies. Frequencymodulations measurements were also made over a range from 20 to 3000 Hz,and gain values were consistently high, with a gain at 3 KHz measure at2×10⁶.

Fabrication and Placement of Nanowires with Registration

1D nanowire photodetector devices of the invention and integration toCMOS circuits of the invention require that individual nanowires beplaced in predetermined, registered positions and the groups ofnanowires have predetermined, registered positions relative to eachother and to electrodes, such as printed circuit electrode patterns.Methods of the invention provide the ability to place individual andgroups of nanowires in such predetermined, registered positions.

The nanowires used in methods to provide predetermined, registeredplacement of the invention can be grown by any of the methods that maybe developed for fabrication of nanowires and of those that arecurrently available, including gas phase syntheses with or without ametal nanoparticle as catalysts using Chemical Vapor Deposition (CVD),Metal Organic Chemical Vapor Deposition (MOCVD), Molecular and ChemicalBeam Epitaxy (MBE and CBE), solution syntheses, template-assistedelectrochemical syntheses, etc. After growth, nanowires are removed fromthe substrates (e.g., by ultrasonication) and dispersed into a liquid(i.e. organic solvents, water etc.). The choice of alloy composition incompound semiconductor nanowires leads to 1D nanowire photodetectors orsegments sensitive to designed colors (light wavelength), such as thoseillustrated in FIGS. 3A-3D.

Single nanowire heterostructures, such as core-shell or core-multishellheterostructures that are doped differently in the core and shell forcreating the majority carrier core 14 and minority carrier shell 16 inFIG. 1 can be created by radial growth and doping steps. Similarly, core14 and radial shell 16 nanowires with different semiconductor materials,including materials having specific spectral response as in FIG. 3D aregrown radially from a core nanowire. Core and shell materials can becontacted individually after positioning of the nanowire by selectiveetching of the shells. Also, alternatively, software analysis of thesingle 1D nanowire response could be employed to isolate colorinformation from the measurement of correlated parameters.

Alternatively, nanowires can be fabricated using nanofabrication methodsincluding ebeam lithography, nanoimprinting lithography, microcontactprinting lithography, focus ion beam lithography, depp UV or x-raylithography, LIGA, etc. followed by etching down using wet chemistry(wet etching) or physical (ion milling, reactive ion etching) etc.

There are a number of prior nanowire alignment techniques, but these donot allow the precise registered and relative control of the resultingnanowire location on substrate. A preferred embodiment method for suchregistration is shown in FIG. 4. In FIG. 4, a substrate is pre-treated40 for nanowire adhesion. The pretreatment 40 can include, for example,deposition of metallic “sticking pads” or other chemicals that canfacilitate nanowire adhesion to the substrate. The substrate is thencoated with photoresist patterned 42 with trenches that approximatelycorrespond to the diameters of the nanowires that are to be registeredand aligned on the substrate. The trenches can be formed usinglithographic methods such as photolithography, e-beam lithography,nanoimprinting lithography, and microcontact printing lithography.Nanowires are aligned in a dispersion 44. The alignment can beaccomplished by a number of previous techniques that provide for theparallel alignment of nanowires in a dispersion on the surface ofliquid, e.g. i.e. water. In a preferred embodiment, theLangmuir-Blodgett technique is used along with nanowire surfacemodification via one or more surfactants. The nanowires are thentransferred 46 onto the substrate that was pre-patterned withphotoresist (step 2), for example by dipping the substrate into theLangmuir-Blodgett trough and using the standard Langmuir-Blodgetttransfer technique or Langmuir-Schaeffer technique. The photoresist isthen removed 48, e.g., using standard developing methods, resulting inthe placement and registration of the nanowires on the substrate in thepredefined locations. The nanowire positioning error is around 250 nm²,primarily limited by the resolution of photolithography.

The placement precision relates to the trench dimension and shape. Thetrenches need to have a certain width so that the nanowires can fall inwith minimum positioning variations. For example, a 2 μm long, 50 nmdiameter nanowire requires a minimum trench width of 200 nm toaccommodate 10% angular misalignment in the Langmuir-Blodgett process.In this case, the FIG. 4 method will produce a maximum positioning errorof 100 nm. For a trench width of greater than 0.5 μm that can be easilyachieved with conventional UV lithography, the registration error isless than 200 nm for 100 nm diameter nanowires. In the direction of thewires, the interdigitated electrodes essentially define the devicegeometry.

Other alignment techniques can also be used in step 44. Using fluidicalignment or electropheretic alignment or contact transfer is possible,for example. For fluidic alignment, the nanowires are dispersed intosolution, such as methyl or ethyl alcohol; and the flexiblemicrochannels used for alignment (e.g., PDMS channels) are aligned withthe trenches in the photoresist during transfer. When the fluid iscirculated in the channels the nanowires can fall into photoresisttrenches. For electrophoretic or magnetophoretic alignment, thesubstrate with photoresist trenches are immersed under the nanowiredispersion solution and the nanowires can fall into the trenches uponapplication of the electric or magnetic field.

An alternative method for registered placement of individual 1Dnanowires for formation of 1D nanowire photodetectors of the inventionis illustrated in FIG. 5. In FIG. 5, pre-treatment of the substratecreates hydrophobic areas 52 or hydrophilic areas. The substrate is thencoated with photoresist and trench patterns are defined 52 usinglithographic methods, e.g., photolithography, e-beam lithography,nanoimprinting lithography, microcontact printing lithography, etc. Thenanowires are aligned and dispersed 54, preferably by Langmuir-Blodgetttechniques. The nanowires are then transferred onto the substrate 56,such as by the Langmuir-Blodgett transfer technique orLangmuir-Schaeffer technique.

After nanowires are individually placed in predetermined registeredpositions, standard techniques can be used to make patterned contacts tothe nanowires and form 1D nanowire photodetectors that can be part of aCMOS circuit, for example. After being positioned into predeterminedregistered positions, the nanowires can be connected to the CMOScircuitry platform by additional photolithography and metallizationsteps. The metallization can also be combined with the existing metalpads (“sticking pads” patterned onto the CMOS platform to promoteselective bonding).

Additional embodiments of the invention provide nanowire photodetectorshaving a vertical geometry. Such photodetectors include axial and radialheterostructures. Referring now to FIG. 6A, a vertical single nanowirephotodetector is shown having the nanowire photoconductor/nanowireheterostructure 12 disposed between a contact electrode 18 a and atransparent contact electrode 18 b. FIG. 6B shows a vertical nanowirephotodetector array including a two-dimensional array of nanowirephotoconductor/nanowire heterostructures 12 disposed between the contactelectrode 18 a and the transparent contact electrode 18 b. A transparentfilling material 60 is also disposed between the contact electrode 18 aand the transparent contact electrode 18 b.

Nanowire photodetectors as provided herein may be used for a variety ofapplications. As nonlimiting examples of additional applications,hyperspectral imagers can be provided, where a nanowire photodetectorarray or matrix can be utilized in conjunction with a scanningspectrometer. As another example, the RGB embodiments provided hereincould be extended to an arbitrary number of wavelengths (colors) toacquire simultaneously spatial and spectral information of an image.Examples include, but are not limited to, UV sensitive solar blindsensor arrays for space application or fire detection in a minefield,etc., IR sensors (e.g., for use in night goggles), etc.

Another example type of device and/or application includesphototransistor structures. In such a phototransistor structure in FIG.6C, a third terminal (such as a metal electrode 18 c+dielectric material18 d) acts as a gate to tune the photosensitivity (gain) or the responsetime of the nanowire photodetector.

It will also be appreciated that nanowire positioning methods accordingto embodiments of the present invention can be used for applicationsbeyond the example photodetectors shown and described herein.Nonlimiting example applications include nanowire field effecttransistors, nanowire memories for information storage, nanowire logicgates, metallic nanowires for electrical interconnects, nanowire lightemitting diodes and laser diodes, optical intrachip interconnects,nanowire waveguides, gas sensor matrices for electronic noses, nanowirechemical sensors, nanowire biosensors, nanowire mechanical sensors,nanowire MEMS devices, actuators, photovoltaics, nanowirepiezoelectrical devices, etc. to be integrated to CMOS circuits, Siphotonics, etc. Preferably, for the fabricated lateral and verticalnanowires, nanowire as a function device is positioned during thefabrication processing and no further placement is needed.

While specific embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

Various features of the invention are set forth in the appended claims.

The invention claimed is:
 1. A photodetector device for a CMOS imagesensor, comprising: a 1D nanowire; and electrodes individuallycontacting said nanowire in a position to apply a longitudinal electricfield in the 1D nanowire, wherein said 1D nanowire comprises a freephotogenerated carrier core and a trapped photogenerated carrier shellhaving different material properties to promote creating an intrinsicradial electric field.
 2. The device of claim 1, wherein said differentmaterial properties are realized with a radial doping profile thatresults in a different doping in the majority carrier core and theminority carrier shell.
 3. The device of claim 1, wherein said differentmaterial properties are realized with different materials in themajority carrier core and the majority carrier shell.
 4. A photodetectorfor a CMOS image sensor, comprising: a plurality of color filters, aplurality of 1D nanowires respectively aligned and registered with saidplurality of color filters; a plurality of sensing electrodesrespectively contacting said plurality of 1D nanowires and a commonelectrode commonly contacting said plurality of 1D nanowires.
 5. Aphotodetector device for a CMOS image sensor, comprising: a plurality ofcolor filters; a plurality of 1D nanowires respectively registered withrespect to one another and respectively functionalized to separatelysense different wavelengths of light; a plurality of sensing electrodesrespectively contacting said plurality of 1D nanowires and a commonelectrode commonly contacting said plurality of 1D nanowires.
 6. Aphotodetector device for a CMOS image sensor, comprising: a plurality ofcolor filters; a plurality of segments of a 1D nanowire, the segmentsbeing registered with respect to one another and respectivelyfunctionalized to separately sense different wavelengths of light; aplurality of sensing electrodes respectively contacting said pluralityof segments and a common electrode commonly contacting said plurality ofsegments.
 7. The device of claim 6, wherein said plurality of segmentscomprises a plurality of axial segments.
 8. The device of claim 6,wherein said plurality of segments comprises a plurality of radialsegments.
 9. The device of claim 1, wherein said electrodes are axiallyseparated on said 1D nanowire.
 10. An image sensor circuit, comprising:a plurality of photodetector devices of claim 1, registered inpredetermined positions relative to one another and to CMOS imagingcircuitry on a substrate.
 11. A phototransistor, comprising: thephotodetector device of claim 1; and a terminal electrically coupled tosaid electrodes, said terminal providing a gate.
 12. A photodetector,comprising: a contact electrode, a transparent contact electrode; and avertically disposed 1D nanowire disposed between said contact electrodeand said transparent contact electrode, said contact electrode and saidtransparent contact electrode individually contacting said verticallydisposed 1D nanowire in a position to apply a longitudinal electricfield in the 1D nanowire; wherein said vertically disposed nanowirecomprises a free photogenerated carrier core and a trappedphotogenerated carrier shell having different material properties topromote creating an intrinsic radial electric field.
 13. Thephotodetector of claim 12, wherein said vertically disposed 1D nanowirecomprises a plurality of vertically disposed 1D nanowires arranged in anarray.
 14. The photodetector of claim 12, further comprising: anelectrode matrix, wherein said vertically disposed nanowire is addressedby said electrode matrix to provide an image sensor.
 15. Thephotodetector of claim 13, further comprising: an electrode matrix,wherein said plurality of vertically disposed 1D nanowires is addressedby said electrode matrix to provide an image sensor.
 16. The device ofclaim 1, further comprising row and column CMOS circuitry for readingcharge from said 1D nanowire.